Antilock control system

ABSTRACT

The wheel speeds are measured by wheel speed sensors and are fed to an evaluation circuit. By way of the evaluation circuit, brake pressure control signals at least for increasing pressure and for decreasing pressure for a specific duration are then produced for the closed-loop/open-loop control of the wheel movement behavior. Moreover, brake pressure control signals are also usually output by the evaluation circuit in order to keep the brake pressure constant. These brake pressure control signals are fed to brake pressure control units for varying the brake pressure. The core of the invention consists in the fact that the duration at least of the first decrease in brake pressure at the front wheels is selected to be dependent on the speed of the vehicle at the start of braking.

BACKGROUND OF THE INVENTION

The invention relates to an antilock control system (ABS) in whichmeasured wheel speeds are fed to an evaluation circuit which producesbrake pressure control signals for increasing and decreasing brakepressure in stages having specific durations. In these known systems, itis possible that, given full application of the brakes for only a shorttime, the braking distance may in certain conditions be longer than in avehicle without ABS.

SUMMARY OF THE INVENTION

The invention is based on an antilock control system for motor vehicles,in which the wheel speeds are measured by means of wheel speed sensorsand are fed to an evaluation circuit. By means of the evaluationcircuit, brake pressure control signals at least for increasing pressureand for decreasing pressure for a specific duration are then producedfor the closed-loop/open-loop control of the wheel movement behavior.Moreover, brake pressure control signals are also usually output by theevaluation circuit in order to keep the brake pressure constant. Thesebrake pressure control signals are fed to brake pressure control unitsfor varying the brake pressure. The core of the invention consists inthe fact that the duration at least of the first decrease in brakepressure is selected to be dependent on the vehicle speed at the startof braking.

The advantage of the invention consists in the fact that the brakingdistance is shortened for full braking starting from a relatively lowspeed. The invention therefore prevents lengthening of the brakingdistance in vehicles with ABS, in comparison with vehicles without ABS,given full braking starting from a relatively low speed.

In particular, there is provision according to the invention for thedecrease in brake pressure to be influenced at the front wheels, butsuch influencing of the decrease in brake pressure is also possible atthe rear wheels.

In an advantageous embodiment of the invention there is provision forthe duration of at least the first decrease in brake pressure to beselected to be additionally dependent on the determined grip of theunderlying surface.

During the first control cycle of the ABS, the utilization of thebraking power is significantly less efficient than in the subsequentcontrol cycles. This is because in the case of full application of thebrakes, due to occurrence of slippages, the controller must cope with ahighly non-steady-state process at the start of control and because thefeedback necessary for optimization is only available in the followingcontrol cycles. If the entire process of full application of the brakesis relatively short, the first control cycle (or cycles) goes intostrong braking, as a result of which the resulting braking distance maybe longer than in the case of braking without ABS. This appliestypically if full braking takes place at a relatively low starting speedon an underlying surface with good grip.

The invention tolerates the fact that the steerability may be reduced inthe first control cycle (or cycles); the driving stability is howeverretained completely. The following estimation shows that, under thegiven circumstances, shortening the braking distance is more importantfor avoiding an accident than optimum steerability.

At an initial speed of 40 km/h and with a deceleration of 10 m/sec², atime with reduced steerability of approximately 1 sec results; in thistime, little can be achieved by means of a steering maneuver.

In particular, there is provision according to the invention for theduration of at least the first decrease in brake pressure which takesplace within the first control cycle to be selected to be longer athigher vehicle speeds at the start of braking than at lower vehiclespeeds at the start of braking.

In a simple embodiment of the invention, the duration has a fixed valueif the speed of the vehicle is lower at the start of braking than aselectable first threshold value. If the speed of the vehicle is higherat the start of braking than the selectable first threshold value, theduration can be selected independently of the speed of the vehicle atthe start of braking, that is to say the duration of the decrease inbrake pressure takes place according to the known ABS control algorithm,that is to say without the reduction according to the invention.

If the duration is selected, as already mentioned, to be additionallydependent on the determined grip of the underlying surface, thedetermination of the grip of the underlying surface advantageously takesplace by means of an evaluation of the speed of the vehicle. Here, thegrip of the underlying surface is derived from the change of the speedof the vehicle over time.

For the taking into account, according to the invention, of thecondition of the underlying surface, there may be provision in a simpleembodiment of the invention for the duration of the decrease in brakepressure to have a fixed value if the speed of the vehicle is lower atthe start of braking than a selectable first threshold value and,additionally, the determined grip of the underlying surface is higherthan a selectable second threshold value. Here, the duration is alsoindependent of the speed of the vehicle at the start of braking if thespeed of the vehicle at the start of braking is higher than theselectable first threshold value or if the determined grip of theunderlying surface is lower than the selectable second threshold value,that is to say that the duration of the decrease in brake pressure takesplace according to the known ABS control algorithm, that is to saywithout the reduction according to the invention.

The coefficient of adhesion can be derived as a measure of grip from theincrease in the speed (V_(ref)) of the vehicle.

Advantageously, the aforesaid first threshold value for the speed of thevehicle lies in the region from 0 to 60 km/h at the start of braking andthe second threshold value for the coefficient of adhesion whichrepresents the grip of the underlying surface lies in the region of 0.5.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a diagram of a first embodiment,

FIG. 2 is a diagram of a second embodiment,

FIG. 3 is a diagram of a third embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1, the wheel speed sensors 1 to 4 supply signals to anevaluation circuit 5 which forms brake pressure control signals fromthem in accordance with the wheel movement behavior and thus actuatesbrake pressure control units 6, 7, 8 and 9. For the sake of simplicity,only the brake pressure control units for the front wheels areillustrated here in the form of inlet valves 6 and 7 and outlet valves 8and 9. However, it is to be noted carefully that the invention alsorelates to the decrease in brake pressure of the rear wheels. Areference speed V_(ref) which is approximated to the speed of thevehicle is formed in the evaluation circuit 5 from the wheel speedsignals.

The switch 10 is activated at the start of ABS control or when an ABScontrol is triggered. In the simplest case, the switch can also beconstructed as a brake light switch. With the activation of the switch10, a comparator 11 is also activated, the said comparator 11determining whether at that moment the reference speed or the speed ofthe vehicle is lower than a first threshold value S1 (e.g. 40 km/h) andthen outputting a signal. At the same time, the signal of the brakelight switch 10 sets a monostable element 12 into its second position inwhich it outputs a signal for a time of e.g. 0.3 sec. The response ofthe first outlet valve 8 or 9 produces a signal on a line 13. If thisoccurs before the 0.3 sec have expired, an AND gate 14 becomesconductive.

Optionally, in the case of the condition of the underlying surface beingadditionally taken into account, the reference speed V_(Ref) is fed to afurther block 15, after which the increase in this reference speed isdetermined in this block, the said increase being a measure of the gripμ of the underlying surface. The grip μ of the underlying surface isthen fed to a comparator 16 which compares the grip of the underlyingsurface with a second threshold value S2. If, as a result of thecomparator 16, a vehicle deceleration is obtained which indicates a highμ [e.g. μ>(S2=0.6)] (or a high adhesion k, for example k>0.6) a signalis also transmitted from this branch to an AND gate 17.

The AND gate 17 becomes conductive when all the inputs conduct a signal.In this way it is indicated that conditions are present in whichshortening duration of the drop in pressure, that is to say a reductionin the decrease in pressure, to a selectable duration seems to beappropriate. This signal is fed to a timing element 18 which, after adelay time of e.g. 12 ms, outputs a signal which sets a monostableelement 19 with a time constant of e.g. 6 ms. Thus, 12 ms after thetriggering of the outlet valve, and for a time of 6 ms, a signal issupplied to AND gates 20 which prevent the outlet valves 8 and 9 frombeing triggered during this time.

Thus, the decrease in pressure, the beginning of which on the line 13starts the process described above, is interrupted after 12 ms, whichbrings about a reduced decrease in pressure. A signal which only occursin the first control cycle, that is to say only during the firstdecrease in pressure, at a terminal 21 causes the decrease in pressurewhich is shortened to the duration of 12 ms only to occur in the firstcontrol cycle, that is to say only during the first decrease inpressure.

While the first exemplary embodiment shows a simple design of theinvention, an extended second embodiment will be described withreference to FIG. 2.

In FIG. 2, essentially the same blocks with the same functions can beseen as in FIG. 1. Therefore, a repetition of the description will bedispensed with. Differences from the first exemplary embodiment are thechanged blocks 11', 16' (optionally) and 18'.

In the changed block 11' and optionally in the changed block 16', thevalue of the speed V_(ref) of the vehicle or the value of the conditionμ of the underlying surface at the start of the braking process aredetermined. On the output side of these blocks, there is then in eachcase a signal which represents the size of these signals and is fed tothe AND gate 17, in a manner already described, and to the timingelement 18'. On the output side of the timing element 18', a signal isnow present at the monostable element 19 after a delay time has expired.The essential feature of this exemplary embodiment is that by means ofthe timing element 18' the delay time is selected to be dependent on thevalue of the speed V_(ref) of the vehicle or of the value of thecondition μ of the underlying surface at the start of the brakingprocess.

This ensures that the duration of the first decrease in brake pressurecan be selected to be longer for example at higher speeds V_(ref) of thevehicle at the start of braking than at lower speeds V_(ref) of thevehicle at the start of braking. Thus a finer application is possiblewith respect to the conflict of objectives between a short brakingdistance and steerability in that a higher priority can be accorded tothe steerability of the vehicle as the speed of the vehicle increases.

In FIG. 3, essentially the same blocks with the same functions as inFIG. 2 can be seen. A repetition of the description will therefore alsobe dispensed with here. Differences from the first exemplary embodimentare the new blocks 22 and 23 and the changed block 18".

As in the first exemplary embodiment, the triggering of the first outletvalve 8 or 9 produces a signal on a line 13. This signal is nowadditionally fed to a counter 23. The counter 23 counts the triggeringpulses of the first outlet valves 8 and 9 and transmits correspondingsignals to the comparator 22 and optionally to the timing element 18'.In the comparator 22, the counting signal is compared with a thresholdZ. Until the threshold Z is reached, a signal is present at the AND gate17 on the output side of the comparator 22. This permits the following:

While in the first and second exemplary embodiments described only theduration of the first decrease in brake pressure is limited over timeaccording to the invention (terminal 21), in the third exemplaryembodiment the influencing of the duration of the decrease in brakepressure according to the invention can also take place for the furtherperiods of decrease in brake pressure. Thus, for example by setting thethreshold value Z=3, it is possible to influence the duration for thefirst two periods of decrease in brake pressure.

In addition to the extension of the invention, described with respect tothe first and second exemplary embodiments, beyond the first decrease inbrake pressure, there is also optional provision for the output signalsof the counter 23 to be fed to the timing element 18", by means of whichsignals are transmitted to the monostable element 19 as a function ofthe counter state (output signal of the counter 23) after differentdelay times. This permits the duration of the periods of decrease inbrake pressure to become longer as their number increases.

We claim:
 1. Method for controlling brake pressure at the wheels of amotor vehicle, said method comprisingmeasuring speeds of the wheels,determining when braking begins, determining a vehicle speed V_(Ref)when braking begins, determining slippage at each wheel based at leaston said wheel speeds, producing at least one brake pressure reductionsignal for each wheel where slippage occurs, said at least one signalhaving a duration and including at least a first reduction signal, theduration of said first reduction signal being dependent on said vehiclespeed V_(Ref) when braking begins, and decreasing brake pressure at eachwheel where slippage occurs in stages having durations corresponding tothe durations of said at least one pressure reduction signal at saideach wheel where slippage occurs.
 2. Method as in claim 1 wherein theduration of said first reduction signal is longer for higher V_(Ref)when braking begins than at lower V_(Ref) when braking begins.
 3. Methodas in claim 1 wherein the vehicle speed V_(Ref) when braking begins iscompared to a first threshold value S1 and the duration of said firstreduction signal has a fixed value if V_(Ref) when braking begins isless than S1.
 4. Method as in claim 1 wherein the vehicle speed V_(Ref)when braking begins is compared to a first threshold value S1 and theduration of said first reduction signal is independent of V_(Ref) whenbraking begins if V_(Ref) when braking begins exceeds S1.
 5. Method asin claim 1 further comprising determining the grip μ of the underlyingroad surface and making the duration of said first reduction signaladditionally dependent on μ.
 6. Method as in claim 5 further comprisingdetermining the reference speed V_(Ref) after braking begins, said gripμ being determined from said reference speed V_(Ref) after brakingbegins.
 7. Method as in claim 6 wherein a change in vehicle speed overtime is determined from said reference speed V_(Ref) after brakingbegins, said grip μ being determined from said change in vehicle speedover time.
 8. Method as in claim 5 wherein said vehicle speed V_(Ref)when braking begins is compared to a first threshold S1 and said grip μis compared to a second threshold S2, and the duration of said firstreduction signal is fixed if V_(Ref) when braking beings is smaller thanS1 and μ is higher than S2.
 9. Method as in claim 8 wherein the durationof said first reduction signal is independent of V_(Ref) if V_(Ref) whenbraking begins is higher than S1 and μ is lower than S2.
 10. Method asin claim 9 wherein S1 is 0 to 60 km/h and S2 is 0.5.